Linear Module

What is a Linear Module

A linear module, also called a linear motion module, linear actuator stage, or single-axis robot, is a complete mechatronic sub-assembly that produces controlled straight-line motion along one axis. Unlike a bare linear guide, which only constrains and supports motion, a module also actuates it. A typical unit packs five elements into one housing: a structural beam (extruded aluminum profile or steel), one or more linear guideways that carry load and moment, a drive element that turns rotary input into linear travel, end-block bearings that support the drive, and a standardized motor-mount flange and coupling. The output is a carriage or table that travels back and forth when a motor is commanded.

The value of buying a module rather than assembling discrete parts is integration. The manufacturer has already matched the screw diameter to the guide size, pre-aligned the rail to the screw axis within a few hundredths of a millimeter, set the guide preload, and verified the assembly travels straight and quiet. A designer bolts the module to the frame, attaches a motor through the prepared flange, and has a working axis. This is why modules dominate machine-building catalogs: they collapse a multi-component sizing exercise into a single part number.

Historically, the building blocks matured before the integrated module did. The recirculating ball screw was patented in the 1920s and refined for machine tools through the mid twentieth century; the profiled rail linear guide with recirculating ball blocks became a standard catalog item from Japanese and German makers in the 1970s and 1980s. Pre-integrated single-axis modules, combining a screw or belt with a matched guide inside one extrusion, became mass-market products in the 1990s as electronics assembly, semiconductor handling, and packaging machinery demanded fast, repeatable Cartesian motion that could be specified from a catalog rather than custom-designed.

In application scale, linear modules span a wide envelope. The smallest precision axes carry a few kilograms over strokes of 25 to 100 mm with micrometre-class repeatability for optics and semiconductor inspection. Mid-size automation modules handle tens of kilograms over 0.5 to 1.5 m for pick-and-place and machine tending. Large belt and rack modules move hundreds of kilograms over spans of several meters for palletizing, woodworking, and gantry welding. No single module covers this entire range, so selection is fundamentally about mapping stroke, load, speed, and accuracy to the right drive type and frame size.

Four engineering attributes determine whether a module fits a job: the drive type (which sets speed, stiffness, and precision), the size and moment rating (which set load capacity), the accuracy grade (which sets repeatability and positioning error), and the duty rating (which sets service life under the real load and speed profile). The rest of this guide walks through each, because a module that looks adequate on a headline thrust number can still be wrong if its moment rating, critical speed, or accuracy grade does not match the application.

Drive Types and Classification

Linear modules are classified first by drive element, because the drive sets the fundamental trade-off between precision and reach. Three families dominate industrial use: ball screw driven, toothed belt driven, and direct linear motor driven. A fourth, rack-and-pinion, appears in very long or heavy gantry axes. Choosing the wrong family is the single most consequential selection error: a belt module can never reach ball-screw repeatability, and a ball screw can never reach belt speed and length, no matter how the rest is specified. The table below compares the three mainstream drive types on the metrics that decide selection.

Ball screw drive uses a precision-ground recirculating ball screw turned by the motor; recirculating balls between the screw and nut convert rotation to thrust with very low friction and almost no backlash when preloaded. This is the high-precision choice, delivering repeatability down to plus-or-minus 0.003 to 0.01 mm with high stiffness and thrust. A preloaded screw also resists back-driving, so a vertical ball screw axis holds position without a brake when power is cut, which belt axes cannot. The limit is critical speed: a long, thin screw whips at high rotation, so practical strokes are usually under about 2 m and top speed is around 1.2 m/s on compact units such as the HIWIN KK series. Larger leads of 20 or 25 mm trade resolution for more travel per revolution.

Toothed belt drive uses a steel-cord-reinforced polyurethane timing belt running over a driven pulley to pull the carriage. Belts decouple speed from length: a belt module reaches up to 5 m/s with strokes that, in families such as the Festo EGC-TB, extend from 50 mm to roughly 8500 mm in a single span. They accelerate hard, cost far less per meter, and tolerate dust and debris better than screws. The price is precision: belt repeatability is about plus-or-minus 0.05 to 0.1 mm because the belt stretches elastically under load and wears over time, and a belt cannot be preloaded to apply or resist rigid force the way a screw can. Belt modules dominate long-stroke pick-and-place, gantry transfer, and packaging where speed and reach beat sub-micron accuracy.

Linear motor drive eliminates the mechanical transmission entirely: the motor forcer rides directly on a magnet track, so there is no screw, belt, or coupling to wear, backlash, or wind up. With a linear encoder closing the loop, these stages reach repeatability down to plus-or-minus 0.001 mm and speeds up to about 10 m/s with very high acceleration and effectively unlimited cycle life on the drive. The costs are price, heat (the forcer dissipates directly into the structure), the need for a high-resolution encoder and amplifier, and the loss of self-holding (a vertical axis needs a brake). Linear motor modules suit semiconductor lithography, flat-panel inspection, and laser processing where throughput and accuracy justify the expense. Rack-and-pinion, a fourth option, is used for spans beyond what belts handle, trading precision for unlimited modular length.

Guide Systems and Accuracy Grades

Inside every linear module, the guide system carries the load and the moments while the drive only provides thrust. Confusing the two leads to undersized selection, because the moment rating of the guide, not the thrust of the screw, often limits how much overhung mass a module can carry. Two guide architectures are common, and the choice interacts with the accuracy grade of the drive.

Profiled rail with recirculating ball blocks is the standard in precision modules. Hardened steel balls recirculate through machined raceways on a ground rail, giving very low friction, high stiffness, and rated load capacity in all four directions plus the three moment axes (pitch, roll, yaw). Preload classes let the maker remove internal clearance and raise stiffness at the cost of friction and life; light preload suits high-speed light loads, heavy preload suits rigid precision work. Some compact modules instead use a U-shaped or double-rail integrated guideway where the carriage blocks slide on raceways machined directly into the extrusion, which raises moment stiffness around the screw axis in a smaller envelope, the approach HIWIN uses on its KK precision axes.

The drive's accuracy grade is governed for ball screws by ISO 3408-3 and the harmonized DIN ISO 3408 and JIS B1192 standards, which bound lead (travel) deviation. The grade is one of the most abused numbers on a datasheet: a low headline price frequently conceals a loose grade. The table below shows the positioning and transport grades with their representative running and travel accuracy, so a buyer can match grade to process.

Two rules follow from this table. First, positioning grades C0 to C5 are normally supplied preloaded so they have effectively zero backlash, while transport grades C7 and C10 are usually not preloaded and carry measurable backlash, which matters for any axis that reverses direction. Second, the grade bounds positioning accuracy, not repeatability: a C7 module can still repeat to plus-or-minus 0.02 mm returning to the same station from the same direction, yet drift well over a tenth of a millimeter in absolute position across a long stroke because of accumulated lead error. Match the grade to whether the process indexes to fixed stations (repeatability matters) or moves to arbitrary coordinates (positioning accuracy matters).

Sizing, Lead, and Standards

Sizing a linear module is a sequence of bounded checks, not a single calculation. The most common mistake is to size on axial load alone and ignore moment, speed limits, and lead, then discover in service that the carriage deflects, the screw whips, or the resolution is too coarse. The chain below, run in order, prevents that. Each step either passes or forces a larger size, a different lead, or a different drive type.

Profile size and moment. Module sizes are named by profile or rail width, for example HIWIN KK30 through KK130, where the number is the carriage width in millimetres. The size sets the guide block size and therefore the dynamic load rating and the three moment ratings. Because most real payloads hang off the carriage with an offset, the limiting check is usually the combined moment load (the sum of pitch, roll, and yaw moment ratios must stay below 1), not the straight axial load. Always compute the moment from the worst-case overhang, including acceleration forces, before picking a size.

Ball screw critical speed and DN value. A rotating screw has a critical (whirl) speed that falls as the screw gets longer and thinner; it scales roughly with screw root diameter divided by the square of the unsupported length. Running near critical speed causes whip, noise, and rapid wear, so makers publish a speed-versus-stroke chart, and the practical operating speed is capped at roughly 80 percent of critical. The DN value (screw diameter in millimeters times rpm) is a second cap from the ball recirculation limit. If the required speed exceeds either limit, the fixes are a larger screw diameter, a shorter or end-supported span, a coarser lead, or a switch to a belt module.

Lead selection. Lead is the linear travel per screw revolution. The table below shows how lead trades speed against resolution and thrust at a fixed motor speed. A coarse 20 or 25 mm lead delivers fast travel and is the choice for transfer and gantry duty; a fine 5 or 10 mm lead delivers finer resolution and higher thrust for precision and vertical lifting. Resolution is the lead divided by the motor and drive counts per revolution, so a finer lead directly improves positioning fineness.

Standards. The governing reference for the ball screw drive is the ISO 3408 series: Part 1 covers vocabulary and designation, Part 3 covers acceptance conditions and acceptance tests (the lead-tolerance grades above), Part 4 covers static axial rigidity, and Part 5 covers static and dynamic axial load ratings and operational life. DIN ISO 3408 adopts these directly, and JIS B1192 is the closely equivalent Japanese scheme that uses the C and Ct grade letters. Linear guide blocks are dimensioned per ISO and maker interchange standards, and motor-mount flanges typically follow IEC 60072 or NEMA frame patterns. For functional rated life, manufacturers compute screw and guide life in revolutions from the dynamic load rating and the actual load spectrum, then apply a service factor of 1.5 to 2 for shock and vibration.

Key Specification Parameters

A linear module datasheet may list twenty or more parameters, but only a handful drive selection. The eight below are the ones to read first: repeatability, positioning accuracy, maximum speed and acceleration, rated and maximum load, moment ratings, rated thrust, maximum stroke, and protection rating. Each is explained so a buyer can compare two datasheets on the same basis rather than on marketing headlines.

Repeatability and positioning accuracy are two different numbers that are frequently confused. Repeatability is the spread of returns to the same commanded point from the same direction, for example plus-or-minus 0.005 mm on a HIWIN KK precision axis. Positioning accuracy is the absolute error to the true coordinate across the full stroke; it is always larger because it sums lead error, thermal growth, and carriage pitch and yaw. Always confirm which one a quoted figure refers to, and whether it was measured unidirectionally or bidirectionally, since bidirectional figures include backlash and are the honest number for any reversing axis.

Speed and acceleration must both be checked. Headline speed is bounded by ball screw critical speed or belt pulley limit; acceleration is bounded by motor torque, inertia, and, for belts, by belt tension and elastic stretch. A module rated for high speed may not reach it over a long stroke once critical-speed derating applies, so read the speed-versus-stroke curve, not just the peak number.

Load and moment ratings come as a set. The axial (push-pull) thrust is what the screw or belt can deliver; the guide carries the perpendicular load and the three moments. For an overhung payload the moment ratings (pitch Mp, roll Mr, yaw My) usually govern, and the combined-load rule requires the sum of the actual-to-rated ratios across all moments to stay below 1. A module can have ample thrust yet fail on moment if the load hangs far off the carriage.

Stroke, duty, and protection. Maximum stroke is set by the drive (ball screws to about 2 m, belts to roughly 8.5 m). Duty rating, expressed as rated life in kilometers or revolutions at a stated load, determines maintenance interval and must be checked against the real cycle, not a nameplate maximum. Ingress protection (IP class) and lubrication interval matter for the operating environment. The list below summarizes the spec-sheet fields and what each one decides.

• Repeatability: spread on return to a point; the headline precision number, specify uni- or bidirectional.
• Positioning accuracy: absolute error over full stroke; bounded by ball screw grade and thermal effects.
• Max speed / acceleration: bounded by critical speed (screw) or pulley and belt stretch (belt); read the curve.
• Rated and max load: continuous versus peak carriage load; pair with the moment ratings, never alone.
• Moment ratings (Mp, Mr, My): the usual limit for overhung payloads; apply the combined-load sum below 1.
• Rated thrust: axial push-pull force from the drive; matters for pressing and vertical lifting.
• Max stroke: drive-limited; screws to ~2 m, belts to ~8.5 m, rack for longer.
• Protection and lubrication: IP class and relubrication interval set environmental fit and maintenance.

Selection Decision Factors

To turn the preceding five chapters into a specific part number, run the decision sequence below in order. Most selection failures come not from a single wrong value but from deciding at the wrong level: picking a frame size before confirming the drive type, or fixing a lead before checking critical speed. These eight steps double as a fixed RFQ template.

1. Drive type first: Decide ball screw, belt, or linear motor from the precision-versus-reach trade-off. Need plus-or-minus 0.01 mm or better, high stiffness, vertical self-hold, or stroke under 1.5 m, choose ball screw. Need long stroke, high speed, or low cost per meter, choose belt. Need sub-micron precision at very high throughput, choose linear motor. Get this wrong and no later step recovers it.
2. Stroke and travel: Set required stroke plus over-travel and homing allowance. Confirm a single span covers it (screws to ~2 m, belts to ~8.5 m); beyond that, plan a gantry of parallel axes or rack-and-pinion.
3. Load and moment: Compute worst-case carriage load and the moment from the payload offset, including acceleration. Size the profile (for example KK40 to KK100) so the combined moment ratio stays below 1, not just the axial load.
4. Speed, acceleration, and critical speed: Set the motion profile, then verify it against the speed-versus-stroke curve and, for screws, against critical speed and DN value. If it fails, step up screw diameter, shorten the span, coarsen the lead, or switch to belt.
5. Lead and resolution: Pick lead to balance speed against resolution and thrust. Coarse (20 or 25 mm) for fast transfer, fine (5 or 10 mm) for precision and vertical lifting. Confirm resolution meets the process tolerance.
6. Accuracy grade and feedback: Choose ball screw grade (C3 to C5 for precision, C7 for transport) per ISO 3408 and decide whether process tolerance needs only repeatability or true positioning accuracy. For absolute-coordinate work, add a linear encoder for closed-loop feedback.
7. Motor and protection interface: Confirm the motor-mount flange matches the chosen servo or stepper (IEC or NEMA frame, coupling or gearbox), and set IP class and lubrication for the environment (IP54 indoor, higher for washdown or dust).
8. Total cost of ownership: Purchase price plus motor and drive, installation and alignment, lubrication and relubrication labor, and downtime risk. A module that saves cost upfront with a C7 screw, light preload, and short rated life often costs more within three years in scrap, recalibration, and replacement.

One dimension buyers routinely overlook is manufacturer serviceability: local spare-parts inventory, availability of replacement carriages and screws, lubrication-fitting standardization, alignment service, and documentation quality. These seem irrelevant at purchase but determine repair response after five to ten years of production. HIWIN, Bosch Rexroth, Festo, THK, and IAI maintain regional spare-parts and service networks, which makes them safe choices for long-life machines, while the lowest-cost suppliers may not stock matched replacement parts, turning a worn screw into a full-module replacement.